Touch screen keyboards are commonly used in a wide variety of fields. Touch screen keyboards are generally used with a cathode-ray tube (CRT) display in which rows of light emitters are positioned along two edges of the CRT display and rows of light detectors are positioned along the remaining two edges of the CRT display opposite respective light emitters. Designations corresponding to keyboard functions are then displayed on the CRT while each light emitter is selectively energized. As each light emitter is energized, the associated light detector is sampled to determine if light from the emitter is being blocked from reaching the detector. A keyboard function is selected by touching the corresponding designation thereby blocking light from being coupled from a light emitter to the associated light detector. The position of a finger, pointer or the like is detected in the x and y axis by scanning the light emitter/detector pairs along both the x and y axis.
Conventional touch screen keyboard systems must address several problems in order to perform satisfactorily. Typically, touch screen systems rely upon common emitter and detector amplifier electronics, requiring each emitter/detector pair to be matched to within a narrow margin so as to present uniform operating characteristics and thus reliable beam break detection. Further, a light detector may mistake an ambient light source as emanating from its associated light emitter. Under these circumstances, the touch screen keyboard system will be unable to detect the selection of a key. The effects of ambient light can be reduced by modulating the signal applied to the light emitters and then either filtering or synchronously demodulating the signal from the associated light detector. While these conventional techniques are usually adequate to reject most types of ambient light interference, they are often unable to reject interfering light generated by other touch screen keyboards or other frequency modulated devices operating in the vicinity since the interfering light will have the same or similar spectral content as the light generated by the light emitters. Also, the use of a filter to reject ambient light interference can adversely effect the response time of the touch screen keyboard. A narrow band filter needed to adequately reject ambient light require a long excitation period (ring up time) before the filtered signal from the detector can respond to the modulated light generated by the associated light emitter. As a result, the use of narrow band filters necessitates a relatively slow scanning speed.
Another problem associated with conventional touch screen displays is the varying sensitivity of the light detectors depending upon the intensity of ambient light. Most detectors used in touch screen keyboards are phototransistors. Phototransistors have the characteristic of variable gain depending upon the level of ambient light. In order to compensate for these variations in detector gain, touch screen keyboard systems generally overdrive the light emitters so that sufficient light will reach the phototransistors under worse case conditions. Also, the gain of conventional light detectors, as well as light emitters, may also be affected by temperature. Overdriving the emitters can produce serious problems when the detector gain is not in its worse case condition. Under these circumstances, sufficient light from the emitters may be reflected from adjacent surfaces, such as the screen or detector/emitter mounting structure, that the light detector will be incapable of detecting a break in the direct light path from the emitter to the associated detector. Under these circumstances, the touch screen will be unable to detect the selection of a keyboard function. In addition, variations in the electrical properties of the emitters or detectors, dust on the emitter and/or detector surfaces, and mounting misalignments may also effect the coupling of light from the light emitter to the associated light detector.
There is therefore a need for a touch screen display scanning system that is capable of rejecting ambient light interference without overdriving the light emitters.